1. Field of the Invention
The present invention relates to a ferroelectric memory, and particularly to a method for manufacturing a non-volatile memory using a ferroelectric capacitor.
2. Description of the Related Art
In recent years, attention has been given to a semiconductor memory (ferroelectric memory) using a ferroelectric capacitor, as a non-volatile memory. A ferroelectric has the property of holding polarization in the direction of voltage application even though the voltage is eliminated (it has spontaneous polarization). Therefore, the ferroelectric memory can be used as the non-volatile memory. Also the reverse speed of polarization of the ferroelectric is on the order of nanoseconds. Further, the voltage necessary for polarization inversion is also restricted to about 2.0V by optimization of a method for manufacturing a ferroelectric thin film. Due to these reasons, the ferroelectric memory is remarkably excellent in rewriting speed and operating voltage as compared with other non-volatile memories such as a flash memory, an EEPROM (Electrically Erasable Programmable Read-Only Memory), etc. Further, since the number of times that data stored in a ferroelectric memory is rewritable or reprogrammable, is greater than or equal to 1012 times, the ferroelectric memory is now placed on practical use as a RAM (Random Access Memory).
In order to ensure long-term reliability, the ferroelectric memory needs to hold degradation produced due to an imprint to the minimum. The imprint means a phenomenon in which when data is being stored or held in the ferroelectric memory over a long period after writing of the data into the ferroelectric memory, the property of holding data opposite to the held data is degraded. This imprint occurs because when given data is retained, an internal electric field in the same direction as polarization occurs due to redistribution of floating charges lying inside a capacitor.
Since the imprint is relevant to the existence of floating charges lying inside the ferroelectric thin film of the ferroelectric capacitor, it depends on the floating charges that exist in the ferroelectric thin film. In addition to it, the imprint greatly depends on degeneration (damage) of crystalline suffered due to hydrogen or the like introduced into the ferroelectric thin film in a manufacturing process subsequent to the formation of the ferroelectric capacitor. Thus, in order to form the ferroelectric memory, there is a need to provide such a process as not to damage the ferroelectric thin film in the process subsequent to the formation of the ferroelectric capacitor.
As one example therefor, for example, a method for providing a cover film such as Al2O3 immediately after the formation of a ferroelectric capacitor and preventing the occurrence of damage so as not to allow hydrogen or the like to reach a ferroelectric thin film has already been executed. However, the method using the cover film becomes complex in process even in the case where consideration is made from the viewpoint that another film called “cover film” which is not required originally, is added.
As another example, recovery anneal for improving a ferroelectric characteristic of a ferroelectric capacitor has also been carried out. After the formation of the ferroelectric capacitor, a contact hole is provided by etching to make contact with its corresponding electrode of the ferroelectric capacitor. Since the ferroelectric capacitor is damaged by hydrogen or the like produced upon execution of the etching and formation of an interlayer insulating film by an oxide film, recovery anneal for recovering the damage is carried out in an oxygen atmosphere, for example (see Japanese Unexamined Patent Publication No. Hei 10(1998)-247724). In addition to anneal in the oxygen atmosphere, such recovery anneal was carried out in an ozone atmosphere (see Japanese Unexamined Patent Publication No. Hei 6(1994)-13565), in a nitrogen atmosphere (see Japanese Unexamined Patent Publication No. Hei 8(1996)-8409) or in an atmosphere of air (see Japanese Unexamined Patent Publication No. 2003-324186).
Japanese Unexamined Patent Publication No. Hei 10(1998)-247724 discloses that anneal at 550° C. is further done in an oxygen atmosphere to recover damage generated after the recovery anneal.
Since, however, oxidation of tungsten and aluminum occurs after the formation of a conductor plug formed of tungsten (W) or the like and a metal wring formed of aluminum (Al) or the like, for electrically connecting a MOSFET and a ferroelectric capacitor, and an external circuit, it is difficult to carry out anneal in an oxygen atmosphere at a high temperature.